There are all sorts of rules which apply when you want to photocopy someone else’s document. Today’s invention is an attempt to limit the amount of such copying that can be done.
The vertical spacing between lines of text can, given current print technology, be subtly varied to act as a kind of barcode. These small variations could be undetectable by human observers and yet easily recorded by a photocopier.
Such spacing might form a pattern unique to each book or even each page. When a (networked) photocopier is being used, it could easily identify the ISBN of the document in question (or even an id number of an individual copy) and refuse to make more duplicates than the legal limit.